114 ❯ STEP 4. Review the Knowledge You Need to Score High
a table of vapor pressure versus temperature. Subtract the value found in this table from
the measured pressure (Dalton’s law). Values from tables are not considered to be measure-
ments for an experiment. If you are going to use 0.0821 L atm/mol K for R, convert the
pressure to atmospheres.
The value of V may be measured or calculated. A simple measurement of the volume of a
container may be made, or a measurement of the volume of displaced water may be required.
Calculating the volume requires knowing the number of moles of gas present. No matter how
you get the volume, don’t forget to convert it to liters when using PV = nRT or STP.
The values of P, T, and V discussed above may be used, through the use of the ideal gas
equation, to determine the number of moles present in a gaseous sample. Stoichiometry is the
alternative method of determining the number of moles present. A quantity of a substance
is converted to a gas. This conversion may be accomplished in a variety of ways. The most
common stoichiometric methods are through volatilization or reaction. The volatilization
method is the simplest. A weighed quantity (measure the mass) of a substance is converted to
moles by using the molar mass (molecular weight). If a reaction is taking place, the quantity of
one of the substances must be determined (normally with the mass and molar mass), and then,
through the use of the mole-to-mole ratio, this value is converted to moles.
The values of P, T, and n may be used to determine the volume of a gas. If this volume
is to be used with Avogadro’s law of 22.4 L/mol, the combined gas law must be employed
to adjust the volume to STP. This equation will use the measured values for P and T along
with the calculated value of V. These values are combined with STP conditions (0°C
[273.15 K] and 1.00 atm) to determine the molar volume of a gas.
Combining the value of n with the measured mass of a sample will allow you to calcu-
late the molar mass of the gas.
Do not forget: Values found in tables and conversions from one unit to another are not
experimental measurements.Common Mistakes to Avoid
- When using any of the gas laws, be sure you are dealing with gases, not liquids or solids.
We’ve lost track of how many times we’ve seen people apply gas laws in situations in
which no gases were involved. - In any of the gas laws, be sure to express the temperature in kelvin. Failure to do so
is a quite common mistake. - Be sure, especially in stoichiometry problems involving gases, that you are calculating
the volume, pressure, etc. of the correct gas. You can avoid this mistake by clearly label-
ing your quantities (moles of O 2 instead of just moles). - Make sure your answer is reasonable. Analyze the problem; don’t just write a number
down from your calculator. Be sure to check your number of significant figures. - If you have a gas at a certain set of volume/temperature/pressure conditions and the
conditions change, you will probably use the combined gas equation. If moles of gas
are involved, the ideal gas equation will probably be useful. - Make sure your units cancel.
- In using the combined gas equation, make sure you group all initial-condition quanti-
ties on one side of the equals sign and all final-condition quantities on the other side. - Be sure to use the correct molecular mass for those gases that exist as diatomic
molecules—H 2 , N 2 , O 2 , F 2 , Cl 2 , and Br 2 and I 2 vapors. - If the value 22.4 L/mol is to be used, make absolutely sure that it is applied to a gas
at STP.
TIP